How to use the compound interest calculator
- Enter your starting amount and monthly deposit. Either can be zero.
- Add the yearly interest rate and how often interest is compounded. Banks usually compound daily or monthly.
- Choose the number of years. Optionally raise your deposit each year, for example in line with pay rises.
- Read the result: final balance, how much you paid in, how much is interest, the effective yearly rate (APY) and a year-by-year table.
The compound interest formula
P is the starting amount, r the yearly rate, n the number of compounding periods per year, t the years, PMT the monthly deposit and i the equivalent monthly rate. Interest is earned on earlier interest, so growth speeds up over time.
Example
Start with $10,000, add $300 a month for 20 years at 7 % compounded monthly. You pay in $82,000 and end with about $196,700: roughly $114,700, or 58 %, is interest. Over 30 years the same plan reaches about $447,000, because the last decade adds more than the first two together.
How compounding frequency changes the result
| $10,000 at 5 % for 10 years | Final balance | APY |
|---|---|---|
| Yearly | $16,289 | 5.00 % |
| Quarterly | $16,436 | 5.09 % |
| Monthly | $16,470 | 5.12 % |
| Daily | $16,487 | 5.13 % |
Frequency matters less than people think. The rate, the time and how much you add each month matter far more.
Frequently asked questions
What is the rule of 72?
Divide 72 by the yearly rate to estimate how many years money takes to double. At 6 % it doubles in about 12 years; at 9 %, about 8.
What is the difference between APR and APY?
APR is the stated yearly rate. APY includes the effect of compounding, so it is slightly higher. Savings accounts in the US must show APY.
Does this include taxes and inflation?
No. Interest in a regular account is taxed each year, and inflation lowers what the money buys. Use the inflation calculator to see the real value.
Sources: U.S. SEC Investor.gov compound interest calculator method; FDIC, Truth in Savings (Regulation DD) for APY. Illustrations only, not advice.
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Last reviewed: 2026-11-14. Results are estimates based on the returns you enter, not advice. See our disclaimer.